1. Field of the Invention
The present invention relates to a longitudinal grip force estimating device for a vehicle and to a slip control device for a vehicle using such an estimating device.
2. Description of the Prior Art
A grip force of a driven wheel is a torque which is transmitted from the driven wheel to a road surface. A magnitude of such torque is proportional to a magnitude of an acceleration of the vehicle.
Conventionally, as a longitudinal grip force of a driven wheel of a vehicle, an output from a longitudinal acceleration sensor was used, as it was, or a longitudinal acceleration obtained by differentiating a follower wheel speed (i.e., a vehicle body speed), by time, was used.
A longitudinal grip force estimated by such a conventional technique exhibits a sufficient accuracy when the vehicle travels on a flat road surface. However, the vehicle travels on an ascent road, on which it is difficult for vehicles to accelerate, such a longitudinal grip force indicates a value smaller than an actual longitudinal grip force. As a result, an error, or difference, is produced in the estimated longitudinal grip force, which may make it difficult to accurately control the grip of the driven wheel.